This invention relates to a method for performing nuclear magnetic resonance (NMR) studies. More specifically, this invention relates to an NMR method useful for performing NMR studies in a shorter time, but with a substantial reduction in baseline error effects.
U.S. Pat. No. 3,781,650 discloses a method for reducing interference in the receiver of a spin resonance spectrometer. In accordance with the method, a first set of radio frequency (RF) excitation pulses of one phase angle, and a second set of RF excitation pulses which are 180.degree. out of phase with the first set are generated. The free precession decay signals produced by one set of pulses is subtracted from the free precession decay signals produced by the other set. The signals from the excited sample reinforce, while the interference signals cancel.
The method is disclosed in the above-identified Patent with respect to an NMR analytical spectrometer and, therefore, is not concerned with scan-time reduction and magnetic field gradients effects, as is the case in NMR imaging studies of human subjects. Scan-time reduction is important in the study of human subjects to increase patient throughput in the case of medical application of NMR and to decrease the likelihood of artifacts induced by patient motion. Magnetic field gradients are needed in NMR studies, such as imaging, to encode spatial information into the NMR signal to enable image reconstruction.
U.S. Pat. No. 4,443,760, assigned to the same assignee as the present invention and which is incorporated herein by reference, discloses and claims a method for eliminating interference from spurious free induction decay (FID) signals created by imperfect 180.degree. RF time-reversal pulses utilized to produce NMR spin-echo signals. One way this is accomplished is to phase shift by 180.degree. relative to one another successive excitation pulses and subtracting the resulting NMR signals thereby eliminating the spurious FID signals. This method is also effective in eliminating other error components such as d.c. voltage offsets and residual effects associated with magnetic field gradients. The spurious FID signals, d.c. voltage offsets and residual effects of gradients and vibration of the RF coils used to detect NMR signals will be hereinafter collectively referred to as "baseline error component." Another way to eliminate the effects of spurious FID signals is to phase shift successive spin-echo-producing 180.degree. RF pulses and to add the NMR signals. The spurious FID signals are phase shifted and cancel, while the desired signals reinforce. It is necessary to reduce or eliminate baseline error components, since their presence severely degrades image quality.
Although the above-described technique works satisfactorily, there is a drawback associated with the use of phase-alternated RF excitation of the type described above with reference to U.S. Pat. No. 4,443,760, in that in NMR imaging applications the imaging pulse sequence is repeated with the sign of the 90.degree. RF pulse reversed. In some applications, this may be desirable, since not only is the baseline error removed, but the signal-to-noise ratio is improved. In some systems, notably those utilizing high magnetic fields (1.5 tesla), the signal-to-noise ratio may not require enhancement. In this case, the effects of baseline error components are reduced at the expense of increased data collection time.
Another method which is effective in reducing baseline error components, while shortening data collection time, is disclosed and claimed in U.S. patent application Ser. No. 663,659, filed by G. H. Glover and J. R. MacFall, which is assigned to the same assignee as the present invention. The method of the referenced patent application will be described in greater detail hereinafter. Briefly, however, in accordance with the method, it is recognized that the baseline error component does not change rapidly in the course of a scan. Specifically, the baseline error is a slowly varying function of the applied magnetic field gradients. It is possible, therefore, to duplicate relatively small portions of the overall sequence and use this information to calculate a value for the baseline error component. The method of alternating the sign of the 90.degree. RF pulse (i.e., as used herein to indicate alternation of the phase of the RF pulse by 180.degree.) can be used periodically to calculate an exact baseline error for only some of the measurements, and these can then be used to compensate for errors in other NMR signals. This is an effective method for reducing the effects of baseline error components, although at a decreased (compared to the method disclosed in the aforeidentified U.S. Pat. No. 4,443,760) data collection time due to the need to duplicate portions of the pulse sequence. However, some errors can be introduced by the interpolation used to generate baseline error estimates for the NMR signals for which the baseline was not measured directly.
It is, therefore, a principal object of the invention to provide an accurate method for reducing the effects of baseline error components in NMR images without duplicating portions of the scan sequence, thereby shortening the total data collection time.